1. Field of the Invention
The invention relates generally to compositions and methods for dominant gene suppression. Certain embodiments provide methods for preventing transmission of transgenes in gametes. Certain embodiments comprise pairs of plants in which the phenotype of the parents is suppressed in the progeny. Certain embodiments provide constructs and methods useful for generating fertile parental plants that, when crossed, generate sterile progeny plants, and methods of making and using such transgenes and plants, as well as products of such plants.
2. Background Information
Plant breeding provides a means to combine desirable traits in a single plant variety or hybrid, including for example, disease resistance, insect resistance, drought tolerance, improved yield, and better agronomic quality. Field crops generally are bred by pollination, including by self-pollination (selfing; selfed), in which pollen from one flower is transferred to the same or another flower of the same plant, or to a genetically identical plant, and cross-pollination (crossing; crossed), in which pollen from one plant is transferred to a flower of a genetically different plant.
Plants that are selfed and selected for type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two different homozygous lines produces a uniform population of hybrid plants that can be heterozygous at many gene loci. A cross of two plants, each of which is heterozygous at a number of gene loci, generates hybrid plants, which differ genetically and are not uniform.
Many crop plants, including, for example, maize (corn), can be bred using self-pollination or cross-pollination techniques. Maize has separate male and female flowers on the same plant, located on the tassel and the ear, respectively. Natural pollination occurs in maize when wind blows pollen from the tassels to the silks that protrude from the tops of the ears. Many crop plants, including maize, are grown as hybrids, which generally exhibit greater vigor than the parental plants from which they are derived. As such, it is desirable to prevent random pollination when generating hybrid plants.
Hybrid plants (F1) are generated by crossing two different inbred male (P1) and female (P2) parental plants. Hybrid plants are valued because they can display improved yield and vigor as compared to the parental plants from which the hybrids are derived. In addition, hybrid (F1) plants generally have more desirable properties than progeny (F2) plants derived from the hybrid plants. As such, hybrid plants are commercially important, and include many agricultural crops, including, for example, wheat, corn, rice, tomatoes, and melons. Hybridization of maize has received particular focus since the 1930s. The production of hybrid maize involves the development of homozygous inbred male and female lines, the crossing of these lines, and the evaluation of the crosses for improved agronomic performance. Pedigree breeding and recurrent selection are two of the breeding methods used to develop inbred lines from populations. Breeding programs combine desirable traits from two or more inbred lines, or various broad-based sources, into breeding pools from which new inbred lines are developed by selfing and selecting for desired phenotypes. These new inbreds are crossed with other inbred lines and the resultant new hybrids are evaluated to determine which have improved performance or other desirable traits, thus increasing commercial value. The first generation hybrid progeny, designated F1, is more vigorous than its inbred parents. This hybrid vigor, or heterosis, can be manifested in many ways, including increased vegetative growth and increased seed yield.
Production of hybrid seed requires maintenance of the parental seed stocks because self-crossing of hybrid plants produces progeny (F2) that, like P1 and P2, generally exhibit less desirable characteristics than the F1 hybrid plant. Because the parental plants generally have less commercial value than the hybrids (F1), efforts have been made to prevent parental plants in a field from self-crossing (“selfing”), since such crosses would reduce the yield of hybrid seed. Accordingly, methods have been developed to selfing of a parental plant.
One method for controlling pollination is to use a parental population of plants that are male sterile, thus providing the female parent. Several methods have been used for controlling male fertility, including, for example, manual or mechanical emasculation (detasseling), cytoplasmic male sterility, genetic male sterility, and the use of gametocides. For example, parental selfing in a field can be prevented by removing the anthers or detasseling plants of the female parental (P2) population, thus removing the source of P2 pollen from the field. P2 female plants then can be pollinated with P1 pollen by hand or using mechanical means. Hybrid maize seed generally is produced by a male sterility system incorporating manual or mechanical detasseling. Alternate strips of two maize inbreds are planted in a field, and the pollen-bearing tassels are removed from one of the inbreds (P2 female). Provided that the field is sufficiently isolated from sources of foreign maize pollen, the ears of the detasseled inbred are fertilized only by pollen from the other inbred (P1 male); resulting seed is hybrid and forms hybrid plants. Unfortunately, this method is time- and labor-intensive. In addition, environmental variation in plant development can result in plants producing tassels after manual detasseling of the female parent is completed. Therefore detasseling might not ensure complete male sterility of a female inbred plant. In this case, the resultant fertile female plants will successfully shed pollen and some female plants will be self-pollinated. This will result in seed of the female inbred being harvested along with the desired hybrid seed. Female inbred seed is not as productive as F1 seed. In addition, the presence of female inbred seed can represent a germplasm security risk for the company producing the hybrid. The female inbred can also be mechanically detasseled. Mechanical detasseling is approximately as reliable as hand detasseling, but is faster and less costly. However, most detasseling machines produce more damage to the plants than hand detasseling, which reduces F1 seed yields. Thus neither form of detasseling is presently entirely satisfactory, and a need continues to exist for alternative hybrid production methods that reduce production costs, increase production safety, and eliminate self-pollination of the female parent during the production of hybrid seed.
Another method of preventing parental plant selfing is to utilize parental plants that are male sterile or female sterile. Male fertility genes have been identified in a number of plants and include dominant and recessive male fertility genes. Plants that are homozygous for a recessive male fertility gene do not produce viable pollen and are useful as female parental plants. However, a result of the female plants being homozygous recessive for a male fertility gene is that they are not capable of selfing and, therefore, a means must be provided for obtaining pollen in order to maintain the parental P2 plant line. Generally, a maintainer cell line, which is heterozygous for the male fertility gene, is generated by crossing a homozygous dominant male fertile plant with the homozygous recessive female sterile plant. The heterozygous maintainer plants then are crossed with the homozygous recessive male sterile plants to produce a population in which 50% of the progeny are male sterile. The male sterile plants are then selected for use in generating hybrids. As such, the method requires additional breeding and selection steps to obtain the male sterile plants, thus adding to the time and cost required to produce the hybrid plants.
To overcome the requirement of having to select male sterile from male fertile plants generated by crossing a maintainer plant line with a female (male sterile) plant line, methods have been developed to obtain male sterile plants by expressing a cytotoxic molecule in cells of the male reproductive organs of a plant. For example, a nucleic acid encoding the cytotoxic molecule can be linked to a tapetum-specific promoter and introduced into plant cells, such that, upon expression, the toxic molecule kills anther cells, rendering the plant male sterile. As above, however, such female parental plants cannot be selfed and, therefore, require the preparation and use of a maintainer plant line, which, when crossed with the male sterile female parent restores fertility, for example, by providing a dominant male fertility gene, or by providing a means to inactivate or otherwise inhibit the activity of the cytotoxic gene product (see U.S. Pat. No. 5,977,433).
Additional methods of conferring genetic male sterility have been described including, for example, generating plants with multiple mutant genes at separate locations within the genome that confer male sterility (see U.S. Pat. Nos. 4,654,465 and 4,727,219) or with chromosomal translocations (see U.S. Pat. Nos. 3,861,709 and 3,710,511). Another method of conferring genetic male sterility includes identifying a gene that is required for male fertility; silencing the endogenous gene, generating a transgene comprising an inducible promoter operably linked to the coding sequence of the male fertility gene, and inserting the transgene back into the plant, thus generating a plant that is male sterile in the absence of the inducing agent, and can be restored to male fertile by exposing the plant to the inducing agent (see U.S. Pat. No. 5,432,068).
While the previously described methods of obtaining and maintaining hybrid plant lines have been useful for plant breeding and agricultural purposes, they require numerous steps and/or additional lines for maintaining male sterile or female sterile plant populations in order to obtain the hybrid plants. Such requirements contribute to increased costs for growing the hybrid plants and, consequently, increased costs to consumers. Thus, a need exists for convenient and effective methods of producing hybrid plants, and particularly for generating parental lines that can be crossed to obtain hybrid plants.
A reliable system of genetic male sterility would provide a number of advantages over other systems. The laborious detasseling process can be avoided in some genotypes by using cytoplasmic male-sterile (CMS) inbreds. In the absence of a fertility restorer gene, plants of a CMS inbred are male sterile as a result of cytoplasmic (non-nuclear) genome factors. Thus, this CMS characteristic is inherited exclusively through the female parent in maize plants, since only the female provides cytoplasm to the fertilized seed. CMS plants are fertilized with pollen from another inbred that is not male-sterile. Pollen from the second inbred may or may not contribute genes that make the hybrid plants male-fertile. Usually seed from detasseled normal maize and CMS-produced seed of the same hybrid must be blended to insure that adequate pollen loads are available for fertilization when the hybrid plants are grown and to insure cytoplasmic diversity.
Another type of genetic sterility is disclosed in U.S. Pat. Nos. 4,654,465 and 4,727,219 to Brar, et al. However, this form of genetic male sterility requires maintenance of multiple mutant genes at separate locations within the genome and requires a complex marker system to track the genes, making this system inconvenient. Patterson described a genetic system of chromosomal translocations, which can be effective, but is also very complex. (See, U.S. Pat. Nos. 3,861,709 and 3,710,511).
Many other attempts have been made to address the drawbacks of existing sterility systems. For example, Fabijanski et al., developed several methods of causing male sterility in plants (see EPO 89/3010153.8 publication no. 329,308 and PCT application PCT/CA90/00037 published as WO 90/08828). One method includes delivering into the plant a gene encoding a cytotoxic substance that is expressed using a male tissue specific promoter. Another involves an antisense system in which a gene critical to fertility is identified and an antisense construct to the gene inserted in the plant. Mariani, et al. also shows several cytotoxic antisense systems. See EP 89/401, 194. Still other systems use “repressor” genes that inhibit the expression of other genes critical to male fertility. See, WO 90/08829.
A still further improvement of this system is one described at U.S. Pat. No. 5,478,369 in which a method of imparting controllable male sterility is achieved by silencing a gene native to the plant that is critical for male fertility and further introducing a functional copy of the male fertility gene under the control of an inducible promoter which controls expression of the gene. The plant is thus constitutively sterile, becoming fertile only when the promoter is induced, allowing for expression of the male fertility gene.
In a number of circumstances, a particular plant trait is expressed by maintenance of a homozygous recessive condition. Difficulties arise in maintaining the homozygous condition when a transgenic restoration gene must be used for maintenance. For example, the MS45 gene in maize (U.S. Pat. No. 5,478,369) has been shown to be critical to male fertility. Plants heterozygous or hemizygous for the dominant MS45 allele are fully fertile due to the sporophytic nature of the MS45 fertility trait. A natural mutation in the MS45 gene, designated ms45, imparts a male sterility phenotype to plants when this mutant allele is in the homozygous state. This sterility can be reversed (i.e., fertility restored) when the non-mutant form of the gene is introduced into the plant, either through normal crossing or transgenic complementation methods. However, restoration of fertility by crossing removes the desired homozygous recessive condition, and both methods restore full male fertility and prevent maintenance of pure male sterile maternal lines. The same concerns arise when controlling female fertility of the plant, where a homozygous recessive female must be maintained by crossing with a plant containing a restoration gene. Therefore there is considerable value not only in controlling the expression of restoration genes in a genetic recessive line, but also in controlling the transmission of the restoring genes to progeny during the hybrid production process.